Patent GB-1267511-A (GB)
Title: Improvements relating to edge following apparatus
Inventor: INGLIS MALCOLM KEITH
IPC: G06V30/144
Filed: 1968-02-21
Abstract: 1,267,511. Pattern recognition. EMI Ltd. 16 May, 1969 [21 Feb., 1968], No. 8331/68. Heading G4R. Edge following apparatus includes means for storing digital signals representing values of a parameter at a plurality of points on a matrix, means for analysing signals from a group of such points and producing a location signal in response to a sufficiently large discrete change in value of the parameter, and decision means responsive to a location signal to define a further group of points to be analysed so that successive groups of points tend to follow an edge in the pattern. During a first scan of the pattern by a pick-up tube, the video signal is used to derive a mean level and a deviation signal. During a second scan, the video is digitized by an A-to-D converter using a large number of levels derived from the above mean level signal, and entered into a data store to reconstitute the pattern. The data store is now addressed for read-out to simulate a horizontal rightwards raster scan, and for each stored point the modulus of the difference between it and the average of it and the preceding five points is compared with a function of the deviation signal referred to above to detect a significant change in the stored points (indicating edge reached). The X and Y co-ordinates in the pattern of a point giving such a change are stored in an address store. The 5Î5 array of points centred on this point are read in turn from the data store, the addresses being derived from that in the address store, and the average of the points is taken. The 8 nearest-neighbour points to the given point are now read in turn from the data store using addresses from 8 registers selected in turn by a counter, each point being thresholded with the average from the 5 x 5 array to give 1 or 0. Whether a 1 is to be produced for a value above or below the threshold is determined by the sign of the difference referred to above in the simulated horizontal raster scan. When a 1 first appears in the output of the thresholder, the counter setting which selected the point giving this is coded and used to adjust deflection control circuits which are connected to the addressing circuitry of the data store (and also control the pick-up tube via D-to-A converters), thus moving along the pattern edge. The procedure repeats, entering the co-ordinates of a further edge point into the address store and considering the new 5 x 5 array &c. to get the next deflection circuits adjustment, and so on. In considering the 8 nearest-neighbour points above, the counter selects them going anticlockwise round the given point starting at a point considered most likely to be 0 (when thresholded), the counter setting passed to the coder in a given case suitably resetting the counter for the next case. If, in considering the 8 nearest-neighbour points in a given ease, the thresholder output never goes to 1 (indicating an isolated high-spot), the initial search for the edge is repeated. Alternatively, the influence of high-spots can be reduced by accepting a point as representing a significant change in the simulated raster scan only if both it and at least one of 3 other points satisfy the comparison referred to above, the 3 other points being the point immediately below the given point and the two points flanking this "immediately below" point. To prevent the same edge being followed twice, each pair of co-ordinates being entered in the address store is compared with those already there, equality causing the search for the edge to be repeated. The coder receives its input via a logic circuit capable of storing 4 movements from the counter and which detects 4-point loops, prevents their components from passing to the coder and passes to the coder a movement to break out of the loop. A similar circuit could store 10 movements to deal with larger loops. When the entire area of the pattern has been searched for edges and the edges have been followed, the contents of the address store (the edge co-ordinates) are transferred to a pattern recognition device.